1. Field of the Invention
The invention generally relates to a light guide plate and an associated back light system used for a liquid crystal display (LCD) device.
2. Prior Art
In general, an LCD has the advantages of thinness, light weight and low power consumption. For these reasons, LCDs are widely used in various types of electronic equipment, from pocket calculators to large-scale office automation equipment.
Conventionally, an LCD includes a liquid crystal panel and a back light system for providing illumination. The back light system generally includes a light guide plate, a light source, a reflective plate, a diffusing sheet, and a prism sheet. The light guide plate includes a light incident surface, a light-emitting surface, and a bottom surface opposite to the light-emitting surface. The diffusing sheet and prism sheet are located one on top of the other, on the light-emitting surface. The light source is arranged adjacent to the light incident surface. The reflective plate is arranged under the bottom surface. The light guide plate is commonly wedge-shaped or has a regular oblong configuration. The light guide plate receives light beams from the light source such as a cold cathode fluorescent lamp (CCFL), and outputs the light beams uniformly from a substantially planar surface of the light guide plate. Then, the output light beams are coupled to the liquid crystal panel.
The performance of a display device is often judged by its brightness and uniformity of luminescence. The alternatives for increasing a display's brightness include using more light sources, and/or using brighter light sources. However, additional light sources and/or brighter light sources consume more energy. For portable devices, this decreases a battery's lifetime. Also, adding light sources to the LCD may increase production costs and can lead to reduced reliability of the device.
The brightness of the display can also be enhanced by utilizing the light beams within the LCD more efficiently. That is, by directing more light beams within a predetermined range of viewing angles, and by directing the strongest light beams along a preferred viewing axis within the range of viewing angles. A number of mechanisms have been employed within LCDs to improve the display efficiency in this way. For example, brightness enhancing films (prism films) having prismatic structures are frequently used to direct light beams that would otherwise not be viewed to be within the range of viewing angles. A typical flat back light system may use several different films to provide an overall bright, high contrast display with substantially uniform luminescence within the range of viewing angles. Such kind of conventional back light system is found in U.S. Pat. No. 6,356,391, issued on 2002 Mar. 12.
Referring to FIG. 4, a back light system 10 of the patent includes a light guide plate 16, a light source 12, a reflective plate 24, two prism films 26, 32, and a reflective polarizer 34. Referring to FIG. 5, the prism film 26 has a bottom surface 28, which has an array of prisms defined by a plurality of first prisms 38, second prisms 40 and third prisms 42. Configurations of the three prisms 38, 40, 42 are not the same. Light beams from below impinge on the bottom surface 28 at various inclined angles, traverse through the prism film 26, and emit from a top surface 30 of the prism film 26. The light beams are directed by the prisms 38, 40, 42 to emit at angles substantially parallel to a viewing direction of a liquid crystal panel (not shown). The back light system 10 is complicated, unduly thick, and costly.
Other alternatives for directing light beams along a viewing axis of an LCD have been proposed, such as in U.S. Pat. No. 5,779,337 and U.S. Pat. No. 6,231,200, which issued on 1998 Jul. 14 and 2001 May 15 respectively. The main thrust of the two patents is directly forming prisms on a light-emitting surface of a light guide plate, so as to avoid or decrease the need for separate prism films.
In addition, diffusers are commonly disposed on a light guide plate, in order to mask bright spots in the output of the light guide plate and achieve substantially uniform luminescence. However, most diffusers scatter light beams out from the range of viewing angles, and therefore reduce the brightness within the range of viewing angles. Furthermore, a diffuser generally comprises a mass of diffusing particles therein for scattering light beams passing therethrough. The diffusing particles also partly absorb light beams, resulting in decreased luminescence.